1. Field of the Invention
The present invention relates to a code synchronization in a direct sequence spread spectrum communication system. In particular, the present invention relates to a code synchronization apparatus in a multi-carrier direct sequence spread spectrum communication system and method thereof.
2. Description of the Prior Art
Typically, in a cellular and a personal mobile communication system, a code division multiple access communication method using a direct sequence spread spectrum is adopted. In these communication schemes, processing gain is defined as the ratio of the signal transmit bandwidth to the data rate, which determines a multiple access capacity. Therefore, methods for transferring a signal using wider bandwidth are required in order to increase the data rate with the same processing gain. A broadband CDMA communication is the one of those methods. In the broadband CDMA communication method, there are two methods. First, the method using the single carrier is the one to increase the data rate with the same processing gain by combining the data with the sequence with higher chip rate. To the contrary, the method using the multi-carrier is the one to transfer each of a plurality of carriers each having different frequencies, which modulate data spread by a sequence. In this case, the processing gain at each carrier frequency is less than the first method. However, if the signals received at carrier frequencies are combined coherently, the original processing gain can be restored since signal components are added coherently while noise components are added with random phases. Additionally, in a frequency selective fading channel, there is an advantage that it can obtain a frequency diversity gain. In the multi-carrier broadband direct sequence spread spectrum communication, there is an advantage that it can reduce a power consumption because it does not need a higher device even in processing a baseband signal in contract to the conventional single carrier system that needs to process a much higher baseband signal. Also, since it does not require continuous frequency bands in contrast to the single carrier system, it can increase the efficiency of frequency resources. However, the advantage of the multi-carrier direct sequence spread spectrum system can be obtained only if the phase of each carrier can be known exactly after code synchronization is completely established. Thus, since the phase of each carrier can not be known before the code synchronization is completely established, there is a need for a code synchronization method, which is capable of obtaining the diversity gain mentioned above when the phase of the carrier is not known.
However, a code synchronization system suitable to the characteristics of multi-carrier direct sequence spread spectrum system has not been developed. Only a method used in the single carrier direct sequence spread spectrum system can be employed. Even in this case, since each code synchronization has to be independently performed to the signal modulated into each carrier, there are disadvantages that the time necessary for the synchronization as well as the complexity of the system are increased as much as the number of the carrier used. Also, there is a disadvantage that the diversity gain cannot be obtained because the diversity of the received signal in a fading channel cannot be used.
It is therefore an object of the present invention to solve the problems involved in the prior art, and to provide a code synchronization apparatus of a multi-carrier direct sequence spread spectrum communication system and method thereof which are capable of obtaining a stabilized code synchronization, using a code synchronization method suitable to the situation to adjust the symbol combining gain and the symbol integration time depending on the channel situation.
In order to achieve the above object, the code synchronization apparatus in a multicarrier direct sequence spread spectrum communication system for synchronizing PN code in received signal with reference PN code locally generated in the receiver according to the present invention comprises a plurality of bandpass filter for filtering received signals at each carrier frequency so as to perform a code synchronization of the received signals; a plurality of amplifiers for amplifying the outputs of the bandpass filters; a plurality of automatic gain controlling means for controlling the gain of the amplifiers using the outputs of the amplifiers as inputs, so that the output magnitudes of the amplifiers have a constant value; a pseudo noise code generator for generating pseudo noise codes; a plurality of non-coherent correlators for performing non-coherent detection using the gain controlled signal and the output of the pseudo noise code generators; and a symbol combining and code synchronization test processor for commanding the pseudo noise code generator to generate pseudo noise codes and for combining and testing the output of the non-coherent correlators to determine whether or not the code is synchronized.
The receiver in a multi-carrier direct sequence spread spectrum system for modulating a plurality of carriers of different frequencies into direct sequence spread spectrum signals to transmit them according to the present invention comprises an interface apparatus for viewing surrounding topologies and speed to an user and for generating channel information from the view which is selected by the user, so as to perform in parallel the code synchronization of the received signal; and a system controller for receiving the channel information to calculate an integral time of the non-coherent correlators and the number of symbol to be combined, which are suitable to the channel information, and then commanding them to the each non-coherent correlator and a code synchronization test processor.